This study was initiated to prepare purified erythropoietin in quantities adequate for study of the biochemical and biological features of the molecule, particularly the binding of erythropoietin to early erythropoietic cells and its role in the initiation of specific genetic events. The methods employed have utilized known aspects of erythropoietin structure, especially its high carbohydrate content. Erythropoietin has been partially purified by binding to plant lectins liganded to Sepharose using affinity chromatography. The acidic nature of the erythropoietin molecule has permitted it to be separated from most other proteins by isoelectric focusing techniques. Small amounts of purified erythropoietin may be used to prepare antibodies of high specificity and affinity. These antibodies would be useful in purification of erythropoietin in bulk and in the development of a radioimmunoassay for ESF.